N = int(raw_input())
seq = map(int, raw_input().split())

ans = 0

for (a, b, c) in [(seq[i], seq[i+1], seq[i+2]) for i in range(N-2)]:
    if a == b or b == c or c == a:
        continue
    elif max(a, b, c) == b or min(a, b, c) == b:
        ans += 1
    else: continue

print ans